The use of mineral materials for preparing industrial products for the paint industry, for coating paper, fillers for rubbers and synthetic resins etc. is well known and represented in the prior art. Such mineral materials include, for example, calcium carbonate, sulphate and silicate, as well as aluminum silicate, kaolin, talc and titanium oxides. When used as pigments, such as in paper coatings, solid particles of one or more mineral materials are suspended in water, along with binding agents, dispersing agents, thickeners, coloring agents and, possibly, other additives. Such coatings must have a low viscosity which remains stable throughout the storage, transport and coating processes to facilitate handling and application, as well as the highest possible solids (mineral) content to reduce the amount of heat and time required to dry the coating material.
It is difficult to use some mineral materials, such as the calcium compounds, as pigments because the particles do not naturally disperse, nor remain dispersed, sufficiently. Such mineral substances are typically converted to useful form by grinding into very fine aqueous dispersions in which the constituent particles are as small as possible, i.e., typically smaller than a few microns. Moreover, even when such mineral materials are initially successfully ground and dispersed in an aqueous dispersion, there is often a tendency for the mineral solids to form a gel, which increases the viscosity of the dispersion, sometimes to the point of being useless for the intended application.
To enhance dispersion and reduce gel formation in dispersions of mineral materials, one or more grinding and/or dispersing agents (hereinafter referred to collectively as “dispersing agents”) are often introduced into the mineral dispersion before and/or during grinding. For example, it is well known to use water soluble acrylic polymers and/or copolymers, of low molecular weight, totally or partially neutralized by various neutralizing agents, as the dispersing agents. See, e.g., French Patent Nos. FR2488814 and FR2603042, and European Patent Nos. EP0100947, EP0127388, EP0129329, EP0542644, EP0100948 and EP0542643.
U.S. Pat. No. 4,868,228 discloses grinding agents in aqueous dispersions of mineral materials, and, in particular, the use of such grinding agents to produce low viscosity aqueous dispersions of mineral solids. The grinding agents are described to be acrylic acid polymers and/or copolymers that are completely neutralized by at least one neutralizing agent having a monovalent function and at least one neutralizing agent having a polyvalent function. This reference teaches that the only fraction of acrylic alkaline polymers and/or copolymers of which the specific viscosity lies between 0.3 and 0.8 delivers the maximum required characteristics of a grinding agent. The resulting aqueous dispersions contain at least 70% solids and at least 95% of the mineral particles have a greatest dimension less than 2 microns. French Patent No. FR2488814 discloses a process using acrylic alkaline polymers and/or copolymers for preparing an aqueous dispersion of mineral substances with a high solids concentration of very fine mineral particles of which 95% have a dimension less than 2 microns, and of which 75% at least have a dimension less than 1 micron, and also having a viscosity much lower than that previously obtained. FR2488814 also teaches that the only fraction of acrylic alkaline polymers and/or copolymers of which the specific viscosity lies between 0.3 and 0.8 delivers the maximum required characteristics of a grinding agent. However, it was also observed that the viscosity of the resulting dispersions was not, in fact, stable over time, and that eight days after grinding, the viscosity increased by two to five times the viscosity that had been measured immediately after grinding was completed. U.S. Pat. No. 6,395,813B1 discloses dispersions of solid particles in water formed by grinding using water soluble phosphonate-terminated homopolymer and/or copolymer to form dispersions of fine particles, 70% or more of which are below 2 microns, and 70% or more are below 1 micron. U.S. Pat. No. 6,395,813B1 teaches that such dispersing agents having a weight average molecular weight of from 2000 to 6000 are especially preferred.
It would be advantageous to have a grinding process which produced a mineral dispersion of low initial viscosity, high solids content and having minimal viscosity increases (i.e., minimal gel formation) over time.